I'm not sure what the requirements for your rig radar approach are, but in the GOM there are no NDBs at all. There may be an isolated unit somewhere, but we have no information on any of them, nor do we use them. We use a combination of GPS and radar. GPS is orders of magnitude more accurate than NDB, and gives us very good positioning. We have very accurate coordinates for all platforms and rigs - after all, the rigs use DGPS to locate themselves, so we have positions more accurate than our GPS units can even use. NDBs are being decommissioned in the US very rapidly, and those still commissioned often are NOTAMed OTS. They're going the way of the AN range equipment. I certainly don't claim to know the situation elsewhere.